What is Sealing?
A seal is a device used to prevent the leakage of fluids or gasses between two surfaces. Seals are designed to create a barrier between two parts of a machine or system to prevent the escape of fluids or gasses, or to prevent the entry of contaminants into the system.
Understanding the Types of Mechanical Soft Seals in Trunnion Mounted Ball Valves
Trunnion Mounted Ball Valves (TMBVs) are critical components in various industries like Oil & Gas and Pharmaceutics, due to their ability to control high-pressure and high-flow applications. These valves rely on a variety of soft seals to ensure optimal performance and leak prevention. In this blog, we will explore the different types of mechanical soft seals used in TMBVs, focusing on O-rings, backup rings, valve seats, seat rings and Valve Packings for static applications, and quad rings for dynamic applications. We will delve into their functions, the materials they are made from, and their specific roles within a TMBV.
- O-Rings
Description
O-rings are circular elastomeric rings that create a seal at the interface of two or more parts. When installed, they deform under pressure to effectively seal gaps and mating components. After deflection, O-rings revert to their original shape, maintaining a sealed condition. They are one of the most common types of seals used in various mechanical applications, including TMBVs.
Reasons for Use
O-rings provide an effective and reliable seal with simple design requirements. They are versatile, cost-effective, and easy to install and maintain.
Materials
- Perfluoro elastomer: Excellent resistance to aggressive chemicals such as, hot organic and inorganic acids, caustics, amines especially hot amines >70°C, ketones, aldehydes, esters, ethers, alcohols, fuels, solvents, sour gases, hot water, steam, ethylene propylene oxide and mixed process streams.
- HNBR Rubber (HNBR): Excellent resistance to oils, vegetable oils, greases, glycol based coolants, hydrocarbon oils and sour gases.
- Fluorocarbon (Viton): Excellent resistance to chemicals, hydrocarbon oils, sour gases, steam and high temperature resistance upto 200°C.
- Aflas (FEPM): Good for water and steam applications.
- Fluorosilicone: Excellent resistance to heat, chemical degradation, high tensile and tear strength compared to silicone, excellent low temperature flexibility.
Application in TMBVs
O-rings can be used in both static and dynamic applications:
- Static Seals: These do not move except for pulsation caused by cycle pressure. Examples include axial, radial, dovetail, and boss seals
- Axial: Squeezed axially in the groove like a flat gasket.
- Radial: Squeezed radially between the inside (ID) and outside (OD) of the groove.
- Dovetail: Retained in the face seal during assembly and maintenance.
- Bosstail: Used for sealing straight thread tube fittings in a boss.
- Dynamic Seals: Sit inside a groove between rotating surfaces, providing a leak-tight seal.
In TMBVs, O-rings are commonly used in static applications to seal the valve body, ensuring that no fluid leaks between the valve components. They are placed in the grooves of the Seat Ring and are compressed to form a tight seal.
2. Backup Rings
Description
Backup rings are ring-shaped components made from rigid materials that support O-rings or other seals to prevent extrusion and deformation under high pressure. The two main types of Backup Rings are:
- Scarf Cut Backup Ring
- Solid Backup Ring
- Spiral Backup Ring
- Contoured Rubber Backup Ring
Reasons for Use
Backup rings enhance the performance of O-rings by providing structural support, especially in high-pressure applications. They help maintain the integrity of the seal by preventing extrusion into the clearance gaps.
Materials
- Polytetrafluoroethylene (PTFE): High chemical resistance and low friction.
- Reinforced PTFE (RPTFE): Enhanced mechanical strength and wear resistance.
- Peek: High temperature and chemical resistance with excellent mechanical properties.
- Fluorocarbon (FKM) : High chemical and temperature resistance.
Application in TMBVs
Backup rings are used in conjunction with O-rings in static applications. They are typically placed on the low-pressure side of the O-ring, providing additional support to maintain a reliable seal.
3. Valve Seats
Description
The ball valve seats are critical surfaces responsible for sealing the fluid inside and uniformly distributing the seating stress. In soft seat ball valve designs, either an elastomer or polymer is used as the seal and is inserted into a metallic seat ring. Valve seats interface with the ball and are responsible for stopping the flow of fluid when the valve is closed.
Reasons for Use
Valve seats provide a primary sealing function within TMBVs, ensuring a tight shut-off and preventing fluid leakage. They must withstand the mechanical stress and wear associated with valve operation.
Materials
- PTFE: Offers excellent chemical resistance and low friction.
- Reinforced PTFE (RPTFE): Enhanced mechanical strength and wear resistance.
- Peek: High temperature and chemical resistance with excellent mechanical properties.
Application in TMBVs
Valve seats are located around the ball in the valve body. When the valve is closed, the ball compresses against the seats, creating a tight seal that stops the fluid flow.
4. Seat Rings
Description
The primary function of a ball valve seat ring is to provide a sealing surface against which the ball (the spherical closure element) can press to stop fluid flow. When the ball valve is in the closed position, the seat ring ensures that there is no leakage by creating a tight seal. In the open position, the seat ring ensures minimal resistance to fluid flow. This dual role is vital for the proper functioning of the valve, ensuring both effective shutoff and smooth operation.
Reasons for Use
It forms an essential part of the sealing mechanism that ensures the valve can shut off fluid flow effectively and maintain a tight seal under various operating conditions.
Materials
The choice of material for seat rings depends on the application requirements, including pressure, temperature, and the nature of the fluid being handled. Common materials include:
- PTFE (Polytetrafluoroethylene): Known for its excellent chemical resistance, low friction, and high temperature tolerance, PTFE is a popular choice for seat rings in chemical and pharmaceutical industries.
- Reinforced PTFE (RPTFE): This is PTFE combined with fillers like glass or carbon to enhance its mechanical strength and wear resistance. RPTFE is used in more demanding applications.
- Peek (Polyether Ether Ketone): PEEK offers high temperature and chemical resistance with excellent mechanical properties, making it suitable for high-performance applications.
- Elastomers (e.g., Nitrile Rubber, EPDM): For applications involving lower temperatures and pressures, elastomers provide good flexibility and sealing performance.
- Metal: In some high-pressure or high-temperature applications, metal seat rings are used for their strength and durability. Metal seats can be overlaid with softer materials to improve sealing performance.
Application in TMBVs
In ball valves, seat rings are positioned in the valve body and form a seal with the ball. When selecting seat rings for a ball valve, several factors must be considered:
- Fluid Compatibility: The material must be compatible with the fluid being handled to prevent degradation or chemical attack.
- Temperature and Pressure: The seat ring material must withstand the operating temperature and pressure conditions without deforming or failing.
- Mechanical Wear: Materials should have good wear resistance to ensure long service life, especially in applications involving frequent cycling.
- Regulatory Requirements: In certain industries, seat ring materials must comply with regulatory standards, such as FDA regulations for food and pharmaceutical applications.
5. Quad Rings
Description
Quad rings, also known as X-rings, are four-lobed seals that provide a more stable seal compared to standard O-rings. They are designed to resist rolling or twisting in dynamic applications.
Reasons for Use
Quad rings offer improved sealing performance in dynamic applications due to their unique design, which provides multiple sealing surfaces and reduces friction. They also have better resistance to twisting and extrusion.
Materials
- Nitrile Rubber (NBR): Suitable for oil and fuel resistance.
- Fluorocarbon (Viton): High chemical and temperature resistance.
- EPDM: Suitable for water and steam applications.
Application in TMBVs
In TMBVs, quad rings are used in dynamic applications such as stem seals or shaft seals. They provide reliable sealing under the movement of the valve stem or shaft, ensuring no leakage during valve operation.
Conclusion
Trunnion Mounted Ball Valves rely on a variety of mechanical soft seals to ensure optimal performance and reliability. O-rings, backup rings, valve seats, seat rings, and quad rings each play crucial roles in maintaining a leak-free environment within the valve. The choice of materials for these seals depends on the specific application requirements, including pressure, temperature, and chemical exposure. By understanding the functions and applications of these soft seals, we can appreciate the complexity and precision involved in designing high-performance sealing solutions for TMBVs.
